Digital computers process data in accordance with program instructions. Generally, a computer will retrieve data to be processed from a mass storage subsystem, perform various processing operations in connection therewith to generate processed data, and store the processed data on the mass storage subsystem. The mass storage subsystem may be connected to a plurality of computers to facilitate central storage of the data, and possibly to allow sharing of data thereamong.
Computers are often used in a number of applications in which virtually continuous availability of data is important to the operation of businesses or other entities in which the computers are used. A number of strategies can be used to enhance the likelihood that data is virtually continuously available. A mass storage subsystem can be, for example, mirrored, in which the data contained in a mass storage subsystem is duplicated on another mass storage subsystem either located local to the one mass storage subsystem. This can protect against a failure or other malfunction of the one mass storage subsystem. Alternatively or in addition, another mass storage subsystem can be located remotely from the one mass storage subsystem, which can protect against a catastrophic failure or occurrence that may occur at the location of the first mass storage subsystem. In either case, when the one mass storage subsystem receives a request from a computer to store data thereon, it will generally also transfer the data to the other mass storage subsystem to facilitate storage of the data thereon.